The photocatalyzed degradation of two selected pesticide derivatives, namely 3-tert-butyl-5-chloro-6-methyluraciI (terbacil 1) and 2,4,5-tribromoimidazole (TBI, 2) has been investigated in aqueous suspensions of titanium dioxide (TiO2) under a variety of conditions employing a pH-stat technique. The degradation was studied by monitoring the change in substrate concentration of the model compound employing HPLC analysis and the decrease in total organic carbon (TOC) content, respectively, as a function of irradiation time. The degradation kinetics were studied under different conditions such as reaction pH, substrate and photocatalyst concentration, type of TiO2 photocatalyst and the presence of alternative additives such as H2O2, KBrO3 and (NH4)(2)S2O8 besides molecular oxygen. The degradation rates and the photonic efficiencies were found to be strongly influenced by the above parameters. Toxicity tests for the irradiated samples of the pesticide derivatives measuring the luminescence of bacteria Vibrio fischeri after 30 min of incubation were also performed. The degradation products were analyzed by GC/MS technique and probable pathways for the formation of the products are proposed.